Kernel+LibPthread+LibC: Add a naive futex and use it for pthread_cond_t

This patch implements a simple version of the futex (fast userspace
mutex) API in the kernel and uses it to make the pthread_cond_t API's
block instead of busily sched_yield().

An arbitrary userspace address is passed to the kernel as a "token"
that identifies the futex and you can then FUTEX_WAIT and FUTEX_WAKE
that specific userspace address.

FUTEX_WAIT corresponds to pthread_cond_wait() and FUTEX_WAKE is used
for pthread_cond_signal() and pthread_cond_broadcast().

I'm pretty sure I'm missing something in this implementation, but it's
hopefully okay for a start. :^)

Kernel/Process.h

@@ -233,6 +233,7 @@ public:
     int sys$profiling_enable(pid_t);
     int sys$profiling_disable(pid_t);
     void* sys$get_kernel_info_page();
+    int sys$futex(const Syscall::SC_futex_params*);
 
     static void initialize();
 
@@ -391,6 +392,9 @@ private:
     u64 m_alarm_deadline { 0 };
 
     int m_icon_id { -1 };
+
+    WaitQueue& futex_queue(i32*);
+    HashMap<u32, OwnPtr<WaitQueue>> m_futex_queues;
 };
 
 class ProcessInspectionHandle {